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NZ fern colonises Australia, twice

It is not just people crossing the ditch – a little New Zealand fern has also emigrated to Australia, and not just once but twice.

This is the first known case amongst ferns or seed plants of the same species dispersing twice across the Tasman Sea.

Hooker’s spleenwort fern, or Asplenium hookerianum, is a close relative of the hen & chickens ferns. Hooker’s spleenwort is widespread and common in New Zealand, but rare in Australia, with only a few, small populations in each of Victoria and Tasmania.

DNA analyses of the populations of Hooker’s spleenwort were carried out by researchers from Te Papa, Massey University, and the University of Melbourne.

26 genetic variants were found in New Zealand, but only one each in Victoria and Tasmania. Not only are the Australian variants at the tips of the genetic family tree, they are more closely related to variants in New Zealand than to each other.

Many plant species are known to have dispersed across the Tasman Sea, in either direction. Numerous New Zealand species also occur in Australia (about 50% in ferns), and more have close relatives there. But, it remains an open question how common multiple dispersals within a species are.

As we know, there is a precedent set for dispersal of fern and fern and allies globally by wind, and that is the plant commonly known as Bracken (fern). It has been theorised that Bracken has such small spores, that they may enter the upper atmosphere and use the jet stream to move vast distances across the world finally falling to terra-firma to colonise somewhere new. Well that some where new was a long time ago as the species or genus has long been present in our world – several hundred million years to sure.
If not wind dispersal of micro-spores over long distance, wind dispersal over shorter distances? It is conceivable and it has likely been argued at some point, that while all major land masses were clumped (an official geology term) together – as Pangaea – that like many other species both plant and animal, Bracken may have been able to disperse over the giant land mass over time. The Bracken – as with many other species being shared with different continents could have been an established occupant of those land masses that made up Pangaea before they began drifting apart.
This could be seen as a corollary to the Asplenium (hookerianum) in question.

Thanks for the comment Grant. I think long-distance dispersal on the wind by fern spores is common (on a geological time scale). Fern spores are tiny – c. 0.04 mm – and can be produced in vast numbers.
Of the c. 90 fern species that occur both in Australia and New Zealand, most belong to groups that evolved only after the separation of the landmasses, so there must have been a crossing of the Tasman Sea. Other vectors may have been involved, but spores blowing on the wind seems likely.